1. Field of the Invention
The present invention relates to a fuel control system with improved fuel feed control, particularly for turbine-engines.
2. Summary of the Prior Art
Fuel control systems for turbine engines are known which comprise a flow regulator having its upstream side connected to the output of a proportioning pump and its downstream side to an injection device, a control valve mounted in a by-pass between the pump and the flow regulator, a pressure drop detector intended to provide an output at a modulated pressure Pmod dependent upon the pressure drop Pam-Pav wherein Pam and Pav are the pressures measured upstream and downstream respectively of the flow regulator, the Pmod output acting on the control valve to open or close it when the pressure drop across the flow regulator varies so as to keep the pressure drop substantially constant, an overspeed limiter responsive to the rotational speed of the turbine engine to reduce the injection flow in the user installation when a fixed overspeed threshold is exceeded so as to prevent possible racing of the engine, a stop valve capable of stopping, on command, the flow of fuel injected into the plant, and a control computer for controlling the foregoing components.
It is current practice in this type of control system to provide a control valve mounted in series with the flow regulator, the valve being actuated by a pilot pressure to limit the flow in the event of selected working parameters of the engine exceeding predetermined levels. GB Pat. No. 1 465 480 discloses an example of this type of system in which the pilot pressure is controlled by a torque motor which is actuated by current intensity signals originating from a control circuit which operates in response to the aforesaid working conditions of the engine.
It is also known from FR Pat. No. 2 495 692 to actuate the control valve electrically through torque motors at two levels of excitation, one enabling the valve to retain a mid position authorizing an output flow and the other causing it to assume a position in which the outlet of the valve is sealed and the output flow is therefore nil.
In other arrangements fuel cut-off is ensured by means different from those which provide the fuel regulation, for safety reasons.
These prior systems meet the aim, regarded as desirable, of ensuring that all aspects of the control of the engine, including starting and stopping, are controlled electrically. For this purpose the systems require the presence of control torque motors as well as of an intermediate valve controlling the stop valve.
However, in the interests of reliability of this type of fuel control system for turbine engines, it is desirable that the number of control components utilized be at a minimum.
It is an object of the present invention therefore to improve the reliability of systems of this known type without increasing the hydraulic complexity.
Another object of the invention is to permit starting the turbine engine directly by means of the control computer acting only on the flow regulator by providing the latter with means enabling it to pass from a nil flow position to a position ensuring ignition flow.
A further object of the invention is to provide a control system in which the flow regulator is able to return to the nil flow position only under selected conditions, preferably presenting no danger.